Collision avoidance control has been conventionally performed as one embodiment of driving support, the collision avoidance control being a control for avoiding a collision with a three-dimensional object existing in front of a vehicle or for reducing an impact due to the collision. In order to perform a proper collision avoidance control, it is important to detect a three-dimensional object with a high accuracy, and various apparatuses for detecting a three-dimensional object have been developed lately. For example, the Japanese Patent Application Laid-Open (kokai) No. 2013-161434 discloses an apparatus for detecting a three-dimensional object based on an image taken by a stereo camera.